Optical heterodyne detection systems are utilized to monitor DWDM systems because of their ability to achieve high spectral resolution. Improvements in the performance of optical heterodyne detection systems with regard to parameters such as sensitivity and dynamic range can be achieved by increasing the signal to noise ratio of the heterodyne beat signal. However, the desired heterodyne beat signal coexists with other direct detection signals. The direct detection signals in DWDM systems include intensity noise and shot noise from both the input signal and the local oscillator signal. The various noise components can mask the desired heterodyne beat signal. One technique for improving the signal to noise ratio of the heterodyne beat signal involves reducing the intensity noise by utilizing two detectors to accomplish balanced detection. Although balanced detection is useful in improving the signal to noise ratio of the heterodyne beat signal, balanced detection has limitations.
Another technique for improving the signal to noise ratio of a heterodyne beat signal involves amplifying the input signal before the input signal is combined with the local oscillator signal in order to increase the amplitude of the heterodyne beat signal and thereby increase the signal to noise ratio of the heterodyne beat signal. Although amplifying the input signal increases the amplitude of the heterodyne beat signal, the amplification also increases the intensity noise of the input signal and may not improve the signal to noise ratio of the heterodyne signal.
Another concern with optical heterodyne detection systems is that the receivers are sensitive to surges in optical signal power. For example, the photodetectors can become saturated, or even permanently damaged, when subjected to high signal powers.
In view of the limitations of known heterodyne detection systems, what is needed is an optical heterodyne detection system that generates a heterodyne beat signal with a high signal to noise ratio. In addition, what is needed is an optical heterodyne detection system that can effectively deal with surges of high signal powers.